The present invention relates to wire dot printers, and more particularly to a wire dot printer in which printing is carried out with ink on printing end faces of wires.
A variety of wire dot printers have been extensively employed, almost all of which use ink ribbons. Wire dot printers have been extensively employed, and are advantageous in the following points. The degree of freedom in formation of characters is considerably high; that is, any character or figure can be printed as desired. In addition, characters can be printed as desired with a limited number of dots. Therefore a wire dot printer can carry out printing operations at higher speeds than a matrix type printer. Furthermore, the wire dot printer is advantageous in that a plurality of sheets can be printed simultaneously.
A wire dot printer using an ink ribbon however, has drawbacks in that, since printing is made through the ink ribbon, the resultant prints are not sharp, and since the ink ribbon is struck by the wires, the service life of the ink ribbon is not as long as is desirable. Moreover, if the diameter of the wires is made exceedingly small, because of the great impact stress then imposed on the ribbon, the durability of the ribbon is considerably decreased. In addition to this, it is difficult to make the texture of an ink ribbon as tight as is desirable due to limitations of present cloth weaving techniques. For these reasons, the minimum diameter of wires employed in a wire dot printer using an ink ribbon is limited. That is, it is impossible to use wires of very small diameter with the wire dot printer. Furthermore, it is necessary to provide an additional ink ribbon mechanism for holding and transporting the ink ribbon. This makes the printer intricate and makes it difficult to miniaturize the printer.
The wire dot printer has further disadvantages in the following points. After the wires have been used for a certain period of time, the end faces of the wires tend to become blunt. If the wires have hardened peripheral portions, the print faces of the wires become recessed as they wear. As a result, the stress in striking the ink ribbon is increased until finally the ink ribbon breaks. The length of the ribbon is limited which also limits the service life of the ribbon. Thus, it is difficult to use the same ribbon for a long period of time. Also, in the printer, the mounting position for the ribbon feeding mechanism is limited. Therefore, disadvantageously, the printer is necessarily bulky.
For a multi-pin printer, attempts have been made to decrease the size of characters to be printed. This necessitates an increase in printing density per unit linear distance to provide an acceptable print quality.
In order to fully utilize an advantage of the wire dot printer, namely, a plurality of sheets can be printed simultaneously it is necessary to use a printing force higher than a certain value. Accordingly, if small diameter wires are used, the stress generated upon striking the ribbon with the wires is very high and therefore the ribbon is liable to be broken. On the other hand, in the case of the ink ribbon used in a conventional multi-pin printer, it is impossible to make the texture of the ink ribbon as tight as would otherwise be desirable using presently-available cloth weaving techniques. If the wire diameter is small compared with the texture, then the wire tends to penetrate the ink ribbon as a result of which the wire may be broken. Because of these difficulties, the minimum diameter of the wire is about 0.2 mm where an ink ribbon is used. Accordingly, it is difficult to make imprints having a small line width, high density and high quality. Thus, the quality of print is limited by the wire diameter.
A variety of non-impact printers using no ink ribbon have been proposed in the art. However, such printers suffer from a drawback that a plurality of sheets cannot be printed simultaneously. This is a distinct disadvantage.
Techniques of applying ink to the print end faces of the printing wires without using an ink ribbon have been proposed in the art, for instance, in Japanese Published Application No. 19251/1966, U.S. Application Ser. No. 320,762, U.S. Pat. No. 4,194,846, and German Patent No. 2,546,835. None of these techniques, however, has been found totally satisfactory in that it is difficult to apply a suitable quantity of ink stably and positively to the print end faces of the wires following high-speed repetitive motion of the wire thereby to print sharply and with high quality.
For example, the system disclosed in U.S. Pat. No. 4,194,846, uses a wick material impregnated with ink through which wires run so that top ends (impact faces) of the wires carry ink onto a paper sheet to form a dot pattern. With this technique, it is very difficult to ensure the provision of ink uniformly on each of the top ends of the wires reciprocating at high speed due to a capillary phenomenon in the wick material thereby resulting in non-uniform printing. Further, the wick material tends to clog.
In accordance with German Patent No. 2,546,835, ink from an ink tank disposed below the wires is supplied by capillary action to the wires. With this arrangement, the amount of ink supplied is restricted and thus, the ink supply cannot follow high speed reciprocal operation of the wires causing the printing speed to be limited. Further, there are condensation problems with the ink.